One of the reasons smart phones drain batteries so fast is that they do a lot more than make and receive phone calls. However, cell phones need large-capacity batteries even if you strip away everything they do except for the basic phone functions.

During a phone call the analog information in the sound wave generated by your voice is picked up by the microphone in the phone and converted to a digital signal. On the other end of the call, the digital signal is converted back to analog so the speaker in the phone can produce a sound wave the listener can hear. These conversions back and forth between digital and analog take a lot of power.

The battery-free cell phone does away with these conversions by making use of an analog radio signal emitted by a custom-designed base station. The incoming message is heard when an antenna in the phone picks up a pattern in the radio signal and the speaker in the phone converts the pattern into sound waves. The outgoing message is sent when vibrations in the phone’s microphone caused by the user’s voice are used by the antenna to modify the radio signal emitted by the base station. The station picks up the changed signal and sends it to the person on the other end of the call.

The battery-free phone still needs a 3.5 microwatt power source even though all of the conversions between digital and analog are handled in the base station. If it needs power, it still needs a battery, right? Wrong. The research team devised two methods for drawing power from the ambient environment. Enough power to run the phone can be drawn from the radio frequency signals emitted by the base station or from a grain-of-rice-sized solar cell on the phone’s circuit board.

The base station is doing all the heavy lifting by handling the digital/analog conversions and sending the radio signals that power the phone. The researchers claim this doesn’t pose a problem for implementing their system because the base station’s technology can be integrated easily into cellular networks and WiFi routers.

Credit: Paul G. Allen School University of Washington/YouTube

Using the battery-free cell phone.

This all sounds wonderful, but does it actually work? The researchers demonstrated that it does by making Skype calls with the battery-free phone. Phone numbers are entered with capacitive touch buttons and the user switches back and forth between talk and listening modes with a button press. The phone can communicate with the base station from 31 feet away when powered by radio frequency signals and from a 50-foot distance when using solar power.

At this stage of the game the battery-free phone is an early prototype that’s not ready for prime time. However, the day may not be too far off when your kids roll their eyes at their tech-challenged parents while you mutter, curse and tear the house up looking for your old cell phone charger.

Kevin Murnane covers science, technology and video games for Forbes. His blogs are The Info Monkey & Tuned In To Cycling and he's The Info Monkey on Facebook & @TheInfoMonkey on Twitter.